1. Field of the Invention
The present invention relates to a method and an apparatus for forming an electrostatic latent image on a recording medium by controlling an ionic current in accordance having imagewise exposure through the use of a control grid with a photoconductive layer, and more particularly, to an improved method and apparatus in which an electrostatic latent image is formed on the recording medium by simultaneous charging the medium with an ion current and exposing it to image light.
2. Description of the Prior Art
Among the conventional arts employing a control grid, there are known some methods as disclosed in the U.S. Pat. Nos. 3,645,614 and 3,713,734. According to such prior arts using a control grid A of a four-layer structure, the steps of charging, exposure and ion projection are conducted sequentially, as illustrated in FIG. 1A, 1B and 1C, to form an electrostatic latent image on a recording medium composed of a nonconductive layer and a conductive member superposed thereon, and subsequently a suitable means such as developing device is used to deposit a toner on the latent image to produce a visible image. Since this method effects accurate control of an ionic current in accordance with an optical image, it is possible to obtain a clear image of satisfactory quality with excellent gradation reproducibility.
However, the above methods have disadvantages. Since it is necessary to conduct exposure on the side of the photoconductive layer of a control grid A, as illustrated in FIG. 1A and, after termination of the exposure, to project ion through the control grid A toward the recording medium by a corona charger C or the like, as shown in FIG. 1C, an image projection system must be replaced by the recording medium with respect to the control grid in the ion projection system. Such replacement is particularly disadvantageous in designing a copying machine. Moreover, the above-mentioned sequential process required relatively long time for copy cycle and therefore, is disadvantageous for copy process with high speed. Furthermore, the aforementioned method requires a charger for charging a photoconductive layer and another charger for projecting ions, or requires a means to change, at the time of charging and ion projection, a bias potential applied to the bias layer of a control grid.
For example, in the method disclosed in the Japanese Patent Gazette 45-30320, a plurality of grids are employed to control ions by an electric field between the grids, or by an electric field between the grids and a recording medium, in such a manner that the ions passing through the grids are accumulated on the recording medium under control. For producing a clear image by this method, it is necessary to maintain a considerably higher accuracy with respect to the distance between the grids and also the distance between the grids and the recording medium, or a complicated grid structure is required.
In the method disclosed in the U.S. Pat. No. 3,220,324, charging is performed simultaneously with imagewise exposure by the use of a grid having a photoconductive layer on a conductive screen, so as to control the accumulation of ions on a recording medium. However, it is hard in this case to attain strict control of ions in accordance with an optical image, and an increase of the image contrast is difficult. Moreover, in the methods of the foregoing two patents, the reproduced images are poor in gradation. The present invention provides an improved method which solves all of such problems and can be implemented with facility to constitute a practical apparatus of a relatively simple structure.